Add egg whites, cream of tartar, vanilla, and salt to a stand mixer or bowl
With the whisk attachment beat on medium high speed until soft peaks form
Slowly add in the sugar a spoonful at a time while you continue to beat at high speed
Keep going until stiff peaks form
Transfer the meringue to a piping bag and cut of the tip so there's about a 1/4 inch hole. You can adjust this based on what size you want your mushrooms to be
On a baking sheet lined with parchment paper alternate between piping the caps and stems of the mushroom. To make the caps keep the tip of the pastry bag close to the baking sheet and pipe straight down so that the meringue spreads out into a small domed shape. For the stems start close to the baking sheet and pull upwards as you pipe to create a longer piece of meringue that stand on its own. They're mushrooms so don't worry about making them perfect, they'll all be a bit unique.
Bake at 200 for 4-6 hours. They are done when they feel dried out and are no longer sticky at all. If after 6 hours they are still not done, turn off the oven and leave them overnight. This will dry them out well.
You can let them cool and store them in a sealed ziplock for up to a week. Be very careful of any humidity.
